1. Field of the Invention
The present invention relates to a PCB (printed circuit board) capable of releasing thermal stress, and more particularly, to a PCB or a PWB (printed wiring board) with slits for reducing thermal expansion.
2. Description of the Related Art
Recently, a liquid crystal display panel, a type of flat display, has been widely used as a display for PDAs, laptop computers, TVs, and so on.
As shown in FIG. 1, in a liquid crystal display module adopting the aforesaid liquid crystal display panel, tapes 13 with drive IC chips 14 thereon are used in connecting a liquid crystal display panel 11 and the IC chips 14. Each of TCPs 15 (tape chip carriers), including an IC chip 14 and a tape 13, is connected to either a PWB (printed wiring board) 121 or a PWB 122.
The structure of the PWB 121 gradually changed into a 4-layer board from an early 6-layer board due to reduction in cost. Similarly, the PWB 122 gradually changed from a 4-layer board into a 2-layer board. Unfortunately, the problem of thermal expansion for the PWB 121 or the PWB 122 in dimensions becomes worse than an early one, even though the cost and weight are reduced. The thermal expansion of the PWB 121 or the PWB 122 varies from supplier to supplier no matter whether the circuit layouts are the same, because the materials from various suppliers are different.
Referring to FIG. 2(a), in a thermal compression process, the elongation ΔL of the PWB 121 is in direct proportion to the length L of it, the coefficient of thermal expansion or the temperature increment. The elongation ΔL can be obtained by the following formula:ΔL=Lα(T1−T2)where α is the coefficient of thermal expansion for the PWB 121, T1 is the temperature of thermal compression and T2 is room temperature.
As shown in FIG. 2(b), when ACFs (anisotropic conductive film) are used to connect the input terminals of TCPs 15 and the output terminals of the PWB 121 in a thermal compression process, the connection areas of them are heated to around 150° C. to 200° C. Moreover, the PWB 121, being still hot and expended, is attached to the TCPs 15 which have been already connected and fixed to the liquid crystal display panel 11. Therefore, when the thermal compression process is over and the PWB 121 is cooled down to room temperature, the PWB 121 is bent in accordance with contraction of itself. Consequently, the circuits on the tapes 13 are distorted by the bent PWB 121, and the tapes 13 on the both sides of the PWB 121 are usually damaged due to the distortion.
Now, a PCB or a PWB capable of releasing thermal stress is in high demand because the length of it becomes longer in response to the increment in the size of a liquid crystal display.